The present invention relates to apparatus and method for evaluating the output of a sensor and providing an indication of a measured parameter as a function of the sensor output. More particularly, the present invention relates to the sensing of liquid quantities in a tank using liquid level sensors and providing an indication of the quantity of liquid in the tank as a function of the sensor output.
In sensing the level of a liquid in a container, such as fuel in aircraft wing and fuselage tanks, a plurality of capacitive sensors are mounted in the tanks and the output of the sensors used to provide an indication of the fuel level. In general, the capacitive sensors are formed as concentric, elongated cylinders that function as capacitor plates with the dielectric constant and the capacitance of the sensor varying as a function of the fraction of the sensor wetted by the fuel and the unwetted fraction in the airspace above the fuel/air interface. Capacitor type sensors can be provided with linear characteristics in which the capacitance varies with the fraction of the sensor that is wetted. However, sensors with linear sensing characteristics cannot account for irregularly shaped fuel tanks typically used in aircraft in which the change in quantity of fuel with height is non-linear and can oftentimes vary in a manner approximating a discontinuous, step-wise function. In addition, the attitude of the aircraft fuel tank, for example, whether the aircraft is in level flight with a slight nose-up pitch or on the ground with a slight nose-down pitch, can adversely affect the accuracy of any sensor output.
Various sensor systems have been used to evaluate the output of a fuel level sensor or sensors and provide a corresponding indication of fuel level output. For example, analog systems can introduce various electrical offsets and constants to provide an output that follows a predetermined function to provide fuel quantity information. Additionally, digital systems can digitize the output of the sensors to provide binary data words that can be manipulated arithmetically to provide fuel quantity information. Where the particular shape of the fuel tank or tank system is such that a sensor output that is linear with length is non-representative of fuel quantity, a memory look-up table can be provided in which the sensor output is digitized and forms the basis of a memory address that addresses specific locations in a memory to provide corresponding fuel quantity information. The digitized sensor output, while suitable for addressing a memory look-up table in some situations, is not entirely suitable in situations where a change in sensor attitude relative to the fuel level can provide a sensor output that is no longer representative of fuel level or fuel quantity.